Where can I found information about this feature? I don't know what it does and if I only should add "multilib-sctrict" to my FEATURES in my make.conf.

Thanks a lot for information

https://devmanual.gentoo.org/archs/amd64/index.html wrote:

The multilib-strict Feature

Many Makefiles assume that their libraries should go to /usr/lib, or $(prefix)/lib. This assumption can cause a serious mess if /usr/lib isn't a symlink to /usr/lib64. To find the bad packages, we have a portage feature called multilib-strict. It will prevent emerge from putting 64bit libraries into anything other than (/usr)/lib64.

multilib-strict currently doesn't check perl5, gcc, gcc-lib and eclipse-3, this behaviour is controlled by the MULTILIB_STRICT_EXEMPT variable in make.profile.

How to fix ebuilds properly

In most cases, it's sufficient to use the $(get_libdir) function from multilib.eclass